As known in the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed. When employing twin-wire draw, a group of drying cylinders comprises two closed (endless) wires, fabrics or belts which press the web one from above and the other one from below against heated cylinder faces of drying cylinders arranged in rows. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws which are susceptible to fluttering and may cause web breaks, in particular when the web is still relatively moist and, therefore has a low strength. For this reason, in recent years, ever increasing use has been made of the single-wire draw in which each group of drying cylinders includes only a single closed (endless) drying wire on whose support the web runs through the entire group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces thereof, whereas on the reversing cylinders or rolls between the drying cylinders, the web remains at the side of the outside curve and is subjected to negative pressure as it runs over the reversing cylinders in order to maintain the web on the wire . Thus, in single-wire draw, the drying cylinders are arranged outside the wire loop, and the reversing cylinders or rolls are arranged inside the wire loop.
In so-called normal groups with single-wire draw, known in the prior art, the heated drying cylinders are placed in an upper row and the reversing cylinders or rolls are placed in a lower row below the upper row of drying cylinders, which rows are typically horizontal and parallel to one another. In the following, when the terms "normal (dryer) group" and "inverted (dryer) group" are used, what is meant is expressly groups with single-wire draw in multi-cylinder dryers, of the type mentioned above. In an inverted dryer group, the heated drying cylinders are placed in a lower row and the reversing cylinders or rolls are placed in an upper row above the lower row of drying cylinders.
It is known to those skilled in the art that if paper is dried one-sidedly, the result is a tendency of curling of the sheet. For example, when paper is dried by means of normal groups with single-wire draw from the side of its bottom face only, the drying is asymmetric and if such asymmetric drying is extended over the entire length of the forward dryer section, the drying takes place so that first the bottom-face side of the paper web is dried and, when the drying makes progress, the drying effect is also extended to the side of the top face of the paper web. Under these circumstances, the dried paper is usually curled and becomes concave, when viewed from above.
As known in the prior art, the tendency of curling of paper (or the tendency to curl) is already affected in connection with the web formation, in particular at the sheet formation stage (in, for example, the current assignee's former designated Sym-Former.TM.) by means of the selection of the difference in speed between the slice jet and the wire, and by means of other running parameters. For example, in the case of copying paper, by means of unequalsidedness of drying in the after-dryer, a suitable initial curl form is regulated for the sheet in order that the curling of the paper after one-sided or double-sided copying could be optimized. In the case of copying paper, the reactivity of curling, i.e., the extent to which curling occurs per unit of change in moisture content, is affected to a greater extent by means of a multi-layer structure of the paper, which is produced in connection with the web formation in the wet end.
The most recent technology related to the present invention in high-speed paper machines is based on dryer sections in which there is single-wire draw over the major part of the length of the machine and, with a view toward controlling the tendency of curling of paper, in practice, an inverted group is also almost always used in order to make the drying sufficiently symmetric in the z-direction. However, it has been found that an inverted group has certain obvious, inherent drawbacks in view of the runnability and the overall efficiency of the machine and in view of the profitability of the paper machine investment. Thus, from the point of view of the runnability of the paper machine, a dryer section fully supported over its entire length and based on the use of only normal groups with single-wire draw, without using any inverted groups, would be a highly justified solution. People skilled in the art have, however, not had the courage to introduce this solution in operation, because it has been considered that it would result in solutions uncontrollable and unfavorable from the point of view of the tendency of curling of paper.
One particular problem in the prior art dryer section constructions that include one or more inverted dryer groups is the removal of broke in the event of web breaks, because inverted groups are not self-cleaning by the effect of gravity.
With respect to the prior art related to the present invention, reference is made to the current assignee's Finnish Patent No. 91,900 (corresponding to U.S. Pat. No. 5,416,980 incorporated by reference herein), in which a method is described in the dryer section of a paper machine in particular for reducing the tendency of curling of paper. In this method, the paper web is dried by means of drying cylinders against whose heated faces the paper web is pressed by means of a drying wire. In the dryer section, groups of drying cylinders are used in which twin-wire draw and/or single-wire draw is/are applied. In this method, it has been considered advantageous that in the dryer section, substantially across the entire width of the paper web, hot water steam is fed, by whose means the strains that arise or tend to arise in the fiber mesh in the paper web are relaxed by means of heat and moisture in, or substantially directly after, the area of formation of the strains.
With respect to additional prior art related to the present invention, reference is made further to the current assignee's Finnish Patent No. 93,876 (corresponding to U.S. Pat. No. 5,553,393 incorporated by reference herein) and to the current assignee's Finnish Patent Application Nos. 925942 (corresponding to U.S. Pat. No. 5,465,505 incorporated by reference herein), 935340 (corresponding to U.S. Pat. No. 5,586,397 incorporated by reference herein), 950434 (corresponding to U.S. patent application Ser. No. 08/467,780 incorporated by reference herein) and 951746, and to the current assignee's, as yet unpublished Finnish Patent Application Nos. 963024, 963734, and 963735.
The current assignee's Finnish Patent No. 93,876 describes a dryer section of a paper machine in which there are dryer groups provided with single-wire draw. In this dryer section, it has been considered advantageous that, in order to optimize the drying capacity calculated per unit of length of the dryer section in the machine direction, different ratios of the diameters of the drying cylinders to the diameters of the reversing rolls have been employed with the progress of the drying, i.e., in the running direction of the web, so that in the first group or groups in the initial end of the dryer section this ratio is higher than in the groups in the middle area of the dryer section, and in the group or groups in the final end of the dryer section, a diameter ratio higher than the ratio of the groups at the initial end of the dryer section is used.
The current assignee's Finnish Patent Application No. 935340 describes methods for drying a paper web and dryer sections for a paper web wherein, after the press section, the paper web is dried in a number of successive groups with single-wire draw. According to one exemplifying embodiment, the paper web is dried from the side of its bottom face over the entire length by means of contact drying cylinders, and the paper web is dried from the side of its top face on the draw or draws of the paper web free from the wire, and/or the paper web is dried from the side of its top face by applying a drying air flow through the drying wire to the top face of the paper web.
The current assignee's Finnish Patent Application No. 925942 describes a so-called inverted dryer group with single-wire draw for a multi-cylinder dryer of a paper machine in which a support wire is arranged to be in contact with the reversing cylinders over a substantially large sector, which wire is guided by its guide rolls arranged in gaps between the reversing cylinders and by other necessary rolls, and the web is pressed by means of the tension of the support wire on the sectors against the drying wire.
In the current assignee's Finnish Patent Application No. 951746, a dryer-section concept and a method for drying a paper/board web are described, wherein impingement blowing units or equivalent are arranged in connection with at least some of the drying cylinders. Through the impingement blowing units, a heated medium, preferably air or steam, is passed through the wire into connection with the web so as to produce a two-sided drying effect and to increase the drying capacity.
In the current assignee's Finnish Patent Application No. 963734, a method is described for drying a surface-treated paper web or equivalent in an after-dryer of a paper machine as well as a dryer section of a paper machine for applying the method. With a view toward compensating for a tendency of curling of the paper web, in the after-dryer, the paper web is dried in a dryer group/groups making use of a normal single-wire draw. In connection with or after the drying, the paper web is treated by means of at least one device in order to compensate for a tendency of curling of the paper web, which devices are, for example, a steam box, a blower unit, a moistening device, and/or a soft calender.
On the other hand, in the current assignee's Finnish Patent Application No. 963735, a method for drying a surface-treated paper web or equivalent in an after-dryer of a paper machine as well as an after-dryer of a paper machine for applying the method are described. In the after-dryer, the paper web is dried in at least one dryer group that makes use of single-wire draw, and at the same time, the paper web is dried by means of an impingement drying device arranged in connection with at least one cylinder or roll in that dryer group.
The current assignee's Finnish Patent Application No. 963024 describes a method for drying a paper to be surface-treated, in particular fine paper, in an after-dryer in a paper machine as well as an after-dryer in a paper machine for carrying out the method. After the surface-sizing or coating, the paper web is dried by means of an upwardly open inverted group with single-wire draw, in which connection the tendency of curling formed in the paper web in the forward dryer section can be substantially eliminated and/or compensated for.